“Sometimes our routine actions catch false positives or we make errors. We apologize,” the company’s Public Policy handle tweeted.

Sometimes our routine actions catch false positives or we make errors. We apologize. We’re working today to ensure we overturn any errors but that we remain vigilant in enforcing our rules for those who violate them. As always, account holders can appeal: https://t.co/V3AeH7FdYy

Twitter and other social media platforms are blocked in mainland China under the country’s censorship system, colloquially known as the Great Firewall. But people in China rely on VPNs and proxies to circumvent the blockade.

Activists and government critics, in particular, have taken to Twitter to criticize the nation’s handling of the bloody crackdown on student protestors at Beijing’s Tiananmen Square in 1989.

The violent suppression of the demonstration also led to a tightening of censorship laws across the country, with the Chinese Commmunist Party taking strict measures to forbid any discussion of the pro-democracy movement.

Viewed through this lens, the timing of the ban has raised fresh censorship worries. The suspension provoked a series of strong reactions, including U.S. Senator Marco Rubio, who in a tweet accused Twitter of becoming “a Chinese (government) censor.”

The incident came to light after political activists said a number of accounts were suspended, both inside and outside China.

But Twitter responded that its own spam filters were to blame for the slip-up. It said the suspended accounts “were not mass reported by the Chinese authorities — this was a routine action on our part.” Twitter also said it’s in the process of reinstating the suspended accounts.

However, not everyone is satisfied with the company’s reasoning.

“Twitter‘s explanation is inadequate, if not feeble,” tweeted Yaxue Cao, founder and editor of the US-based publication China Change. “There has to be more going on. Please investigate. Lack of transparency creates uncertainty, fear and conspiracy theories that hurt the company.”